Oil-shale, a sedimentary rock found in vast formations in several parts of the world, contains about 5 to 60 wt. % organic material. About 10% of this organic material is extractable with organic solvents and is called bitumen. The remainder of the organic material is insoluble in organic and inorganic solvents and is a cross-linked macromolecular material called kerogen. The remainder of the oil-shale is inorganic and is composed of various minerals including clays. Much work has recently been done to develop efficient methods for acquiring useful liquids and gases from oil-shale as replacements for petroleum products.
During conversion of oil-shale to liquid products, a significant amount of the mineral matter present in the oil-shale becomes entrained as fine particles in the resulting shale-oil. These fine mineral matter particles are usually present in an amount from about 2 to 30 wt. %, and are undesirable because they are not compatible with subsequent downstream processing. For example, in subsequent hydrotreating, these entrained fine particles have a tendency to poison the catalyst and to clog the reactor bed when present at levels greater than about 500 ppm.
Various techniques have been used in an attempt to lower the levels of these particles. Such techniques include electrostatic precipitation, ultra-filtration, and centrifugation. While these techniques have met with varying degrees of success, they all suffer from serious technical and/or economical disadvantages. For example, electrostatic precipitation requires numerous stages and is highly dependent on the age of the oil. That is, it will generally be effective for freshly produced oil, but not for oil more than as little as a few hours old. Ultra-filtration is very slow and requires constant cleaning and replacement of filters. Centrifugation, because of the large volume of oil, requires numerous vessels and constant maintenance because of mechanical problems.
Another method taught in the art for removing entrained mineral matter from hydrocarbon fuels is the method disclosed in German Pat. No. 722,406 to Pier et al in which a basic (pH&gt;9) ammonium carbonate solution is used to remove mineral matter from an oil derived from brown coal at temperatures of 100.degree.-400.degree. C. Disadvantages associated with such a process include: (1) decomposition of the mineral matter at temperatures greater than 50.degree. C. requires relatively high pressures; (2) undesirable emulsions form at a pH greater than about 9. Therefore, there is a need in the art for an economical process for removing fine mineral matter particles entrained in shale-oil without the disadvantages discussed above.